This was longer than it needed to be, and in my opinion, somewhat mistaken.

The zombie argument is not an argument for epiphenomenalism, it's an argument against physicalism. It doesn't assume that interactionist dualism is false, regardless of the fact that Chalmers happens to be an epiphenomenalist.

Chalmers furthermore specifies that this true stuff of consciousness is epiphenomenal, without causal potency—but why say that?

Maybe because interactionism violates the laws of physics and is somewhat at odds with everything we (think we) know about cognition. There may be other arguments as well. It has mostly fallen out of favor. I don't know the specific reasons why Chalmers rejects it.

Once you see the collision between the general rule that consciousness has no effect, to the specific implication that consciousness has no effect on how you think about consciousness (in any way that affects your internal narrative that you could choose to say out loud), zombie-ism stops being intuitive. It starts requiring you to postulate strange things.

In the epiphenomenalist view, for whatever evolutionary reason, we developed to have discussions and beliefs in rich inner lives. Maybe those thoughts and discussions help us with being altruistic, or maybe they're a necessary part of our own activity. Maybe the illusion of interactionism is necessary for us to have complex cognition and decisionmaking.

Also in the epiphenomenalist view, psychophysical laws relate mental states to neurophysical aspects of our cognition. So for some reason there is a relation between acting/thinking of pain, and mental states which are painful. It's not arbitrary or coincidental because the mental reaction to pain (dislike/avoid) is a mirror of the physical reaction to pain (express dislike/do things to avoid it).

But Chalmers just wrote all that stuff down, in his very physical book, and so did the zombie-Chalmers.

Chalmers isn't denying that the zombie Chalmers would write that stuff down. He's denying that its beliefs would be justified. Maybe there's a version of me in a parallel universe that doesn't know anything about philosophy but is forced to type certain combinations of letters at gunpoint - that doesn't mean that I don't have reasons to believe the same things about philosophy in this universe.